About a month ago I had to spend £30 replacing the lock on the boat after I forgot the key and we had to kick the door open. That was quite annoying. Even more annoying when I have a simple checklist I (almost) always use when packing to ensure I don't forget common items.
Anyway, since it's evolved over several years of travelling, I thought you might be interested to see it in case it inspires you to make or amend your own.
Note it's just a single list suitable for long holidays, Glastonbury or an overnight stay (with headings so I can skip sections that aren't relevant any given trip) - because even I can't be bothered maintaining lots of separate lists with chunks of repetition. It's also not exhaustive, instead focussing on the slightly unusual stuff that I often forget.
You see? Keys even have their own section... £30! Dammit!